This invention relates generally to band saw machines and particularly to swivelable guide heads adapted to be mounted respectively above and below the work table to twist the band saw blade through a range of swiveled positions.
Band saw machines typically have upper and lower pulleys positioned respectively above and below a horizontal work table. An endless band saw blade is trained between the pulleys with the forward, cutting strand trained for vertical downward movement at the forward portion of the work table. A vertical structural column at the back of the work table connects an upper portion of the frame to a base beneath the table. The upper pulley is secured to the upper part of the frame and the lower pulley is secured to the base.
As a rule, the blade cuts sidewise and a standard band saw machine can cut off only comparatively short portions of a work piece limited to the "throat" of the machine. The throat is the fore and aft distance along the work table between the cutting strand of the saw blade and the above-mentioned vertical structural column, or the upwardly moving return strand of the saw blade at the back of the machine. Only very special purpose machines, with oversize throats, which are expensive and bulky, are capable of cutting off substantially long pieces, and the cut-off lengths of these pieces are still limited to the size of the throat.
Attempts have been made to provide pairs of swivelable band saw guides which grip the cutting strand of the blade above and below the work table and are constructed in such a manner that the blade may be twisted to enable a work piece to be cut off without contacting either the back structural column of the frame, or the return strand of the blade.
One example of this is shown in Edwards U.S. Pat. No. 1,783,330 issued Dec. 2, 1930 on a "Saw Guide". However, the saw guide proposed in that patent had severe limitations. The side guide rollers were limited-contact rollers and did not provide sufficiently wide engagement with the sides of a saw blade to twist the blade anywhere close to 90.degree. as is possible with the present invention. Further, in the guide head shown in that prior art patent, there was no provision for optionally adapting the side guide rollers to wide or narrow blade use. In fact, the variety of wide and narrow band saw blades, which are readily available today for cutting metallic and non-metallic materials, was unknown in 1929 when the Edwards patent application was filed. For that reason, an important feature of the present invention, namely the capability of adapting the guide heads for optional use with wide or narrow blades, is not taught by this prior art patent.
A problem which is common to band saws is that they tend to deviate from the desired cutting path, with this tendency increasing as the blade departs more and more from its intended path. One reason advanced for this phenomenon is that the teeth are not manufactured with sufficient accuracy to result in equal cutting forces being produced on both sides of the blade, and thus the cutting edge tends to track to one side or the other of the desired cutting path. As the blade progresses through the material in this abnormal posture, the material itself applies a force to the flat side of the blade which is presenting itself to the oncoming material. This tends to increase the lateral deflection of the blade so that the blade deviation from its intended track is continuously increased. Furthermore, blade deviation is more pronounced at higher feed rates of the material past the blade.
To maintain the desired cutting direction, especially while the blade is twisted as much as 90.degree. as is possible with the present invention, it is important that the side guide rollers in the swivelable guides engage the full width of the blade beginning immediately behind the edge with the cutting teeth.